Surely this depends greatly on what your actual username is?!
It's not uncommon for female users to use the name of a male celebrity whom they fancy admire, which could get very confusing - not least because there is already somebody out there with that name (the celeb himself)!
If you're one of the users who cba to think of anything original, so you just go for User154328965431247585, how would people pronounce or remember it?!
Also, one of the key reasons for having a username on MN in the first place is for anonymity, so you would then have to change to a new username, to prevent it being extremely obvious who you are every time you post.
We'd end up with people irl being called names like ODFOD, NameChangedForThisThread, MumYoureSoEmbarrassing, CantShitPleaseHelpUrgent, IThinkIHaveACocklodger etc. - logically meaning that all MN usernames would then be along the lines of SarahTaylor, RebeccaPerkins and AndreaPeters 